The present invention is related to processing of closed caption data.
Closed caption is displayed on a television screen during a TV broadcast in order to help the hearing impaired to understand the words spoken in the broadcast, or to translate the broadcast to another language, or to display announcements or advertising, or for some other purpose. The processing of the closed caption data is illustrated in FIG. 1. A broadcaster 10 transmits a television signal 12. Signal 12 is picked up by a television receiver 13. Receiver 13 is part of a television set, or is a set-top top box attached to a television set. Receiver 13 extracts the audio data 12A, the video data 12VD, and the closed caption data 12CC from signal 12. The audio data 12A are provided to audio circuit 14. The audio circuit performs suitable processing and provides an audio signal to television speaker 15.
The video signal 12VD is processed by a video decoder 17. The closed caption signal 12CC is processed by a closed caption decoder 16. The outputs of video decoder 17 and closed caption decoder 16 are combined, and the combined signal 22 is provided to a cathode ray tube (not shown) that drives television screen 20. The combined signal 22 causes the television screen to display the closed caption in a window 18. (In FIG. 1, screen 20 contains a human FIG. 24 representing an image carried by video signal 12VD.)
Different standards have been proposed for processing the closed caption data. One such standard, for analog television, is described in Chapter 1 of 47 CFR (Code of Federal Regulations), 1997, incorporated herein by reference. This standard is also known as EIA-608. xe2x80x9cEIAxe2x80x9d stands for Electronic Industries Association. According to the standard, the screen 20 (or, rather, a xe2x80x9csafe caption areaxe2x80x9d of the screen 20) is subdivided into 15 rows. A caption may contain up to four rows L1, L2, L3, L4 (FIG. 2). Each of these rows can be displayed in any one of the 15 rows of the safe caption area. The rows need not be contiguous.
FIG. 3 illustrates closed caption displayed according to a later standard EIA-708 described in xe2x80x9cEIA Standard; Advanced Television Closed Captioning; EIA-708xe2x80x9d (Electronic Industries Association, Engineering Department, July 1997), incorporated herein by reference. This standard (the xe2x80x9c708 standardxe2x80x9d) is designed for digital TV. The 708 standard allows a number of caption windows to be displayed on the screen at any given time. Two windows 18.1, 18.2 are shown in FIG. 3. Each window may have one or more rows.
Each of standards EIA-608, EIA-708 specifies how the caption data should be maintained in a memory (not shown) of closed caption decoder 16. The standards specify under what conditions different portions of the memory need to be erased, and under what conditions the contents of the memory must be preserved. The 708 standard also provides for caption formatted according to EI8-608 if the 608 caption is incorporated into the digital TV signal in a digital form. Hence, the 708 closed caption decoder should be able to decode the 608 closed caption.
It is desirable to reduce the amount of memory needed for a closed caption decoder to decode data transmitted according to different closed caption standards or formats.
Some embodiments of the present invention reduce the amount of memory needed for a closed caption decoder to process caption data according to different standards. In some embodiments, the memory is shared between two different standards or formats. For example, in some embodiments, the memory is organized according to the 708 standard. In particular, the closed caption decoder allocates a memory buffer for each closed caption window, as specified by the 708 standard. Advantageously, these 708 memory buffers are also used to store the 608 caption data.
In some embodiments, the decoder is capable to display a single buffer of up to four caption rows in a single EIA-708 window of a TV screen. The consecutive rows are displayed contiguously, as specified by the 708 standard. However, the 608 standard allows different rows to be displayed non-contiguously in the 15-row safe caption area, as discussed above in connection with FIG. 2. The decoder is unable to display consecutive rows of a memory buffer non-contiguously. Therefore, for some 608 display styles, the decoder stores different 608 caption rows in different memory buffers. This facilitates the non-contiguous display of 608 caption rows since the decoder is not limited to displaying different buffers contiguously.
The ability to display 608 rows non-contiguously is not always needed. The 608 standard specifies three display stylesxe2x80x94pop-on, paint-on, and roll-up. The ability to display rows non-contiguously is needed for the pop-on and paint-on styles, but not for the roll-up style. In the roll-up style, the rows are always displayed contiguously. The text is scrolled up as a new caption line (xe2x80x9crowxe2x80x9d) is received. 47 CFR 15.119(f)(1)(iii) states:
Each time a Carriage Return is received, the text in the top row of the window is erased from memory and from the display or scrolled off the top of the window. The remaining rows of text are each rolled up into the next highest row in the window, leaving the base row blank and ready to accept new text.
Further, 47 CFR 15.119(f)(1)(iii) states: xe2x80x9cThis roll-up must appear smooth to the user. . .xe2x80x9d
The 708 requires a similar capability, that is, a capability to smoothly scroll up a 708 window. Therefore, in the 608 roll-up style, the decoder stores all the rows in the same memory buffer. The result is that at least a portion of the decoder software and/or hardware which implements smooth scrolling up for the 708 rows can be used to implement smooth roll-up for the 608 rows. Using the same software and/or hardware for both tasks reduces the size of the decoder.
According to the 608 standard, when a command is received which necessitates switching from the roll-up style to the pop-on or paint-on style, the text that has been displayed in the roll-up style must remain on the screen and in the decoder memory. However, the pop-on and paint-on styles require non-contiguous display capability. Therefore, in these styles, the decoder stores different caption rows in different buffers as described above. In some embodiments, when a switch is performed from the roll-up style to pop-on or paint-on, the decoder copies all but one rows from the buffer that was used for the roll-up style to different buffers. This copying operation is time consuming.
In some embodiments, the copying is avoided. Different rows are always stored in different buffers in the roll-up style. This is done in addition to storing all the rows in a single buffer. Thus, if the caption has up to four rows, then one buffer stores all the rows, and in addition, up to three other buffers store one row each. The additional buffers are not displayed until switching to the pop-on or paint-on style. If and when the decoder switches to the pop-on or paint-on style, row copying is not needed.
In the roll-up style, the additional buffers are kept up to date when the text is scrolled. Keeping the additional buffers up to date does not require smooth scrolling because the additional buffers are not displayed in the roll-up style.
Other features and advantages of the invention are described below. The invention is defined by the appended claims.